This invention relates to digital signal processing systems and, more particularly, to PCM systems using counting coders.
In counting-type PCM coders, a reference waveform is usually generated. This reference waveform can take the form of a cyclical voltage ramp, perhaps companded, which is compared with successive samples of an input analog signal. At the start of each sampling interval, a counter is activated which generates a count in the form, for example, of an 8-bit digital word, representing the time within the sampling interval at which the input analog sample and the reference waveform correspond.
Idle or quiet channel noise is often a problem in counting-type PCM systems in which analog signals are quantized and digitized. Quantization of the idle channel noise signal produces a signal representing random transitions across a code state boundary. In systems for processing speech signals, this results in an undesirable audible sound.